By Jeff Mason

WASHINGTON (Reuters) - President Barack Obama's re-election campaign regularly rips Republican presidential frontrunner Mitt Romney for his economic policies. On Tuesday, it added former Senator Rick Santorum to its list of top targets.

In a sign of the growing seriousness with which the president's team takes the former Pennsylvania lawmaker, Obama's campaign gave equal billing to Santorum in a memo criticizing the two Republican candidates' tax and deficit reduction plans.

"Governor Mitt Romney and Senator Rick Santorum claim they will champion spending cuts deep enough to cut taxes and balance the budget," Obama campaign policy director James Kvaal wrote in the note, which was distributed to reporters.

"In fact, they have both proposed irresponsible and reckless tax plans that would drive up the deficit by trillions of dollars, while their claims to balance the budget through spending cuts are completely unrealistic."

Santorum has surged ahead of Romney, the former governor of Massachusetts, in some national polls. He beat the struggling frontrunner in a string of recent nominating contests in Missouri, Minnesota and Colorado.

Though Obama's campaign has kept its fire directed primarily on Romney, it has started to watch Santorum closely.

The campaign's Pennsylvania state director said last week in an email blast that Santorum's "extreme-right social views are as out of touch as they are memorable," and asked locals to collect stories that revealed his "true colors."

Santorum has drawn contrasts between himself and Obama on social issues, criticized the president's administration on Monday of implementing healthcare policies that discourage marriage and hurt families.

Adding Santorum to a critique of Romney's economic policies offered a new way for Obama's campaign to attack the surging conservative.

BUDGET CRITIQUE

Romney and Santorum have some similar proposals.

Romney would extend Bush-era income tax cuts for wealthy earners while eliminating education and child-care tax breaks that benefit lower-income taxpayers. His plan would cut annual tax revenue by $180 billion, according to the nonpartisan Tax Policy Center.

"Romney repeatedly promises to balance the budget, but his platform not only fails to reach that goal - it would actually increase the deficit and drive up the debt," Kvaal wrote.

"The cost of his tax cuts and defense increases will total approximately $375 billion in Fiscal Year 2016, the year Romney chose as the benchmark for his budget plans," Kvaal said.

Santorum's tax plan, meanwhile, would dramatically cut tax revenues. His plan calls for collapsing the current six income tax brackets down to two at 28 percent and 10 percent. The plan would keep in place tax breaks for mortgages and healthcare and would boost tax breaks for parents.

"With its even larger tax cuts, the Santorum plan would drive up the deficit even higher. Santorum has proposed $900 billion in tax cuts in 2015," Kvaal said.

Both tax plans have been criticized by conservatives, too.

Conservatives attack Romney's plan for maintaining a tax on investment income for households making more than $200,000 a year, while Santorum has been accused of playing favorites through the tax code by offering benefits to manufacturing businesses and families with children.

(additional reporting by Patrick Temple-West; Editing by Philip Barbara)